Why do we overcrowd?

Why do we overcrowd?

Overcrowding sometimes seems to make economic sense, but at a certain level it hurts your cows and your bottom line.

by Rick Grant
The author is president of the William H. Miner Agricultural Research Institute, Chazy, N.Y.

Rumination, reproduction and milk quality may all suffer in an overcrowded pen. Think again when adding that “last cow” to the group.

We know that on-farm social and group dynamics influence the behavioral and productive response to stocking rate. The evidence is clear that transition cow pens should never be stocked beyond 80 to 90 percent of stalls or feed bunk capacity. The question asked by many producers is, “What are the consequences when I overstock my lactation pens?”

Spanish research found that, as stall stocking density inflated, herd milk production sunk. Stocking density is a key component of the cow’s social environment. It determines if she will meet her time budget requirements for feeding, resting and ruminating and, consequently, be healthy and productive.

Overcrowding alters behavior
Dry matter intake is a function of the number of meals, meal length and eating rate. Ordinarily, our management goal is to encourage more meals with a slower eating rate. But, with limited access to feed and the resulting intensified competition, cows actually consume fewer meals with a greater rate of eating. In a chronic situation, this feeding pattern leads to poor rumen health and reduced feed efficiency.

When stocking density within a pen elevates, the frequency of aggressive interactions surges. Overstocked cows spend less time lying down and more time standing outside the free stall. They consume feed up to 25 percent faster and lie down more quickly after milking.

Competition at the feed bunk is responsible for 88 percent of displacements, indicating that gaining access to feed is a high priority for cows. Competitive success by cows at the feed bunk varies according to each cow’s motivation to eat. In addition to altered feeding behavior, overstocking may also suppress rumination activity, lower milkfat percentage and inflate somatic cell count under some conditions.

Although there is considerable variation among studies, it appears that, beyond about 120 percent stocking rate of stalls, every study has observed a reduction in resting time. Resting is a key behavior for dairy cattle, and accumulating research indicates a requirement for resting of about 11 to 12 hours per day. Cows sacrifice eating time in an effort to recoup lost rest, and they will stand idly in alleys waiting for a free stall to open up rather than eating in overstocked situations.

Studies conducted to date have found that resting time goes down as stocking rate goes up, especially beyond 115 to 120 percent of stalls. There is considerable variability among studies, just as there is variability in response to stocking density among farms. But every study has found losses in resting time beyond 120 percent that range from about 45 minutes to nearly 2 hours. Under these overstocked conditions, cows experience more displacements from the stall just as at the feed bunk. Then, when they return from the parlor, they will often skip feeding and choose to lie down.

We have found that, under conditions of overcrowding, the best single measure of cow comfort in a pen is the stall use index. This index is calculated as the number of cows lying down in a stall divided by the number of standing cows in the pen that are not eating at the bunk. If you think about this index, you will see that it allows you to assess the proportion of cows that are resting versus wasting their time standing in the alleys.

Overcrowding may reduce rumination by 10 to 20 percent. Lactating cows fed a well-formulated diet should ruminate for 8 to 9 hours per day. Stocking densities of 130 to 140 percent of stalls and headlocks have reduced rumination by 1 to 2 hours per day. Studies have found that cows spend less time ruminating while lying in the stalls which is the most effective posture for rumination. We need to think more about how overcrowding interacts with other management factors that are known to reduce rumination such as mixed parity groups, excessive headlock time and heat stress.

Research at Miner Institute found that, as stall stocking density surged from 100 to 142 percent, milkfat percentage was reduced and somatic cell count spiked. In fact, overstocked cows ate 25 percent faster and ruminated 1 hour per day less which explained the reduction in milkfat test. Overstocked cows also experience a greater pathogen load in their environment, have greater teat end exposure to pathogens and may experience immune suppression. These changes could explain the observed negative effect of overcrowding on milk quality.

Stocking density does not cause a change in milk components in all studies, and we need to better understand under what conditions it will alter milk characteristics. There may be an interaction between stocking density and diet. One could easily imagine that a diet higher in unsaturated fatty acids or marginal in physically effective NDF would more readily result in changes in milk components at higher stocking densities.

Wisconsin researchers evaluated data from 153 farms in an effort to identify factors of greatest significance in influencing reproductive performance. Surprisingly, bunk space in the breeding pen rose to the top. These researchers found that, as bunk space dropped from 24 to 12 inches per cow, the percentage of cows pregnant by 150 days in milk halved from 70 to 35 percent. Additionally, reduced conception rates are associated with higher stocking densities.

Subordinate cows hit sooner
Even though research indicates that stall stocking density may approach 120 percent before negative effects on behavior are observed, we have found that first-calf heifers in mixed pens and lame cows are negatively affected at only 113 percent stocking density. In addition, we need to consider the differences in feed bunk stocking density between four- and six-row barns.

The table summarizes the observed changes in cow behavior and the potential economic losses that may result.

Bottom-line stocking density recommendations, for now, are:

Close-up and fresh cows: less than or equal to 80 percent of bunk space (30 inches per cow) which may be a function of stall availability.

Lactating cows in a six-row barn: There is not much data, but conservatively do not exceed 100 percent of stalls since feed bunk is overcrowded by about one-third.

From the cow’s perspective, stocking density is probably less important than ready access to resources such as feed and stalls. We must ensure unhindered access to feed, water and stalls 24 hours a day.

This article appears in the July 2012 issue of Hoard's Dairyman on page 449.